Fc receptor-dependent effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC) constitute an important bridge between innate and adaptive immunities to HIV-1 [1]. They trigger clearing of virus particles or virus-infected cells through mechanisms involving interactions of antibody constant (Fc) region and Fcγ receptors (FcγRs) on the surface of cells involved or potentially involved in HIV infection (natural killer cells (NKs), monocytes, macrophages, dendritic cells, and neutrophils) [2-4].
In contrast to broadly neutralizing antibody responses, which become detectable 2-4 years post-infection [5, 6], ADCC responses appear relatively early during acute infection [7, 8] and are detectable as early as 21 and 48 days after SIVmac251 [9-11] and acute HIV-1 [12] infection, respectively. These early ADCC responses, which in general are broadly reactive, precede the appearance of neutralizing antibody responses with similar breadth [13, 14]. ADCC ultimately leads to the target cell being killed by an effector cell, decreasing the probability of cell-to-cell transmission.
Considerable evidence supports a role of ADCC in preventing or modulating HIV-1 infection. ADCC responses in chronically HIV-1 infected individuals were shown to correlate with slower progression of HIV [15-18] or decreased virus replication [19]. HIV-specific antibodies capable of inducing ADCC and therefore important and serve to protect, for example, against infection in mother-to-infant transmission [20].
Another Fc-mediated effector function, antibody-dependent cell-mediated viral inhibition (ADCVI) coupled with the low-affinity IgG receptor CD16 (FcγRIIIa) genotype, correlated with protection against HIV-1 transmission in a subset of volunteers in the Vax004 vaccine efficacy trial [21]. The picture is even stronger for non-human primate (NHP) infections where sustained ADCC responses predict delayed disease in SIV/SHIV infection [22, 23] and ADCC activity correlates with vaccine-induced protection, often in the absence of neutralizing antibodies [6, 10, 24-28]. Passive immunization studies using a variant of the neutralizing monoclonal antibody, b12, that lacks full Fc receptor binding function but fully retains neutralization potency, resulted in reduced protection against SHIV challenge acquisition [29, 30]. To this end, the latest report on the protective effect of vaginal application of neutralizing and non-neutralizing antibodies against vaginal SHIV162p3 challenge acquisition [31] clearly indicated that mAbs capable of Fc-dependent function provide post-infection control of the SHIV challenge virus, leading to in vivo anti-HIV effects that may strengthen those elicited by neutralizing mAbs.
With regard to vaccine-induced protection in humans, the recent vaccination strategy tested in the ALVACHIV/AIDSVAXB/E RV144 vaccine trial, which showed an estimated vaccine efficacy of 31.2% in preventing HIV-1 acquisition, is of particular importance [32]. It proved, for the first time, that vaccination can protect humans from HIV-1 infection, and that protection could be due to the generation of a blend of neutralizing and non-neutralizing antibody responses with the presence of very modest CD4 T cell responses [32, 33].
The RV144 immunization regimen selectively induced non-neutralizing IgG3 responses which showed highly coordinated Fc-mediated effector responses [34, 35]. These responses correlated with decreased risk of HIV-1 infection in a blinded follow-up case control study [34]. The array of mAbs elicited in RV144 trial was narrow with specificities for the second variable (V2) loop region [33] as well as CD4-inducible epitopes within the first and second constant (C1/C2) domain of gp120 [36]. Whereas V2-specific mAbs were suggested to act through both direct neutralization and Fc-mediated effector function [37, 38], the C1/C2-region specific mAbs were non-neutralizing and capable of potent ADCC [39]. Most importantly, ADCC emerged as a potential correlate of protection in secondary analyses in the subset vaccinated subjects with low plasma anti-HIV-1 Env IgA titers [40]. These studies clearly indicated that RV144 vaccine induced broadly reactive ADCC responses to the C1/C2 domain and that these responses might be partially responsible for the protection observed.
Together, this data suggests an intriguing link between vaccine-mediated protection and ADCC, and provides the first evidence that ADCC might contribute to HIV-1 vaccine efficacy in humans. A role for Fc-mediated effector function in protective immunity against HIV-1 is also indicated. Studies also point toward non-neutralizing, conformational epitopes in the C1/C2 region as important players in these responses (reviewed in [47, 50, 51]). It is well established that this transitional epitope region becomes exposed on the virion spike only upon binding to the CD4 receptor post-infection [48, 49, 52, 53]. In addition, binding to CD4 significantly enhances its exposure in context of monomeric full length gp120 [44]. The fact that the ADCC responses were directed to these transitional CD4-induced epitopes in the RV144 trial consisting of immunization regiment of ALVACHIV vCP1521 prime and recombinant clade B/E monomeric gp120 boost confirms high immunogenicity of this region.
Protein immunogens based on the C1/C2 region of gp120 that have the ability to induce non-neutralizing antibody responses along with Fc-mediated effector responses could serve as effective components of HIV-1 vaccines. The present invention is directed to the elucidation of such peptides as well as other important goals.